Field of the Invention
The present invention relates to an apparatus for detecting variation abnormality in air-fuel ratio among cylinders of a multicylinder internal combustion engine, and in particular, to an apparatus that detects abnormality (imbalance abnormality) in which the air-fuel ratio of one cylinder deviates relatively significantly from the air-fuel ratio of the remaining cylinders.
Description of the Related Art
In general, an internal combustion engine with an exhaust purification system utilizing a catalyst efficiently removes harmful exhaust components using the catalyst and thus needs to control the mixing ratio between air and fuel in an air-fuel mixture combusted in the internal combustion engine, that is, the air-fuel ratio. To control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage in the internal combustion engine to perform feedback control to make the detected air-fuel ratio equal to a predetermined target air-fuel ratio.
On the other hand, a multicylinder internal combustion engine normally controls the air-fuel ratio using identical controlled variables for all cylinders. Thus, even when the air-fuel ratio control is performed, the actual air-fuel ratio may vary among the cylinders. In this case, if the variation is at a low level, the variation can be absorbed by the air-fuel ratio feedback control, and the catalyst also serves to remove harmful exhaust components. Consequently, such a low-level variation does not affect exhaust emissions and pose an obvious problem.
However, if the air-fuel ratio among the cylinders significantly vary since, for example, fuel injection systems for apart of cylinders become defective, the exhaust emissions disadvantageously deteriorate. Such a significant variation in air-fuel ratio as deteriorates the exhaust emissions is desirably detected as abnormality. In particular, for automotive internal combustion engines, there has been a demand to detect variation abnormality in air-fuel ratio among the cylinders in a vehicle mounted state (on board) in order to prevent a vehicle with deteriorated exhaust emissions from travelling.
A possible method for detecting variation abnormality in air-fuel ratio among the cylinders involves calculating a parameter correlated with the degree of variation in output from the air-fuel ratio sensor and comparing the calculated parameter with a predetermined determination value to detect abnormality.
When the output from the air-fuel ratio sensor is utilized, the output characteristic (gain, responsiveness, or the like) of the actually installed air-fuel ratio sensor is advantageously taken into account for improving detection accuracy. Thus, according to PTL1: Japanese Patent Laid-Open No. 2011-47332, it is determined whether or not the output characteristic of the air-fuel ratio is appropriate, and when the output characteristic is determined not to be appropriate, imbalance determination, that is, variation abnormality detection, is inhibited.
When the output characteristic of the actually installed air-fuel ratio sensor is taken into account, an air-fuel ratio varying state may forcibly be generated, and the resulting output from the air-fuel ratio may be utilized.
However, the results of the present inventors' studies indicate that it is insufficient to simply utilize the output from the air-fuel ratio sensor resulting from the forcible generation of the air-fuel ratio varying state. That is, accurate detection of variation abnormality has been found to be difficult to achieve unless an output from the air-fuel ratio sensor obtained before the forcible generation of the air-fuel ratio varying state is also utilized.
Thus, in view of these circumstances, it is an object of the present invention to provide an inter-cylinder air-fuel ratio variation abnormality detection apparatus which also utilizes the output from the air-fuel ratio sensor obtained before the forcible generation of the air-fuel ratio varying state to enable the accuracy of detection of variation abnormality to be improved.